Papers by Steven A. Lloyd
Research Objectives The objectives for this SOLVE project were 3-fold. First, we sought to calcul... more Research Objectives The objectives for this SOLVE project were 3-fold. First, we sought to calculate a complete set of photolysis rate coefficients (j-values) for the campaign along the ER-2 and DC-8 flight tracks. En route to this goal, it would be necessary to develop a comprehensive set of input geophysical Rate Coefficient Calculations in Support of SOLVE Campaign 7
Equatorial Superrotation on Earth Induced by Optically Thick Dust Clouds
Journal of Geophysical Research, 1999
The pattern of seasonal ozone loss over Fairbanks, Alaska (AK), during the NASA Photochemistry of... more The pattern of seasonal ozone loss over Fairbanks, Alaska (AK), during the NASA Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) campaign in the spring and summer of 1997 is defined. Five independent data sets of total ozone observations at Fairbanks are presented, from the Earth Probe and ADEOS Total Ozone Mapping Spectrometer (TOMS) satellite instruments, balloon-borne electrochemical concentration cell ozonesondes, and ground-based (Brewer spectroradiometer, Dobson spectrophotometer, and the Jet Propulsion Laboratory MklV infrared interferometer) instruments. The excellent agreement between different observational techniques lends confidence to the observed rate of summertime loss of total ozone at high latitudes. In addition, the small offsets between the data sets are well understood. ER-2 aircraft enabled the determination of the relative rates
Journal of Geophysical Research: Atmospheres, 1997
Measurements of nitric oxide (NO), nitrogen dioxide (NO2), and total reactive nitrogen (NOy = NO ... more Measurements of nitric oxide (NO), nitrogen dioxide (NO2), and total reactive nitrogen (NOy = NO + NO2 + NO3 + HNO3 + C1ONO2 + 2N20 5 + ...) were made during austral fall, winter, and spring 1994 as part of the NASA Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft mission. Comparisons between measured NO2 values and those calculated using a steady state (SS) approximation are presented for flights at mid and high latitudes. The SS results agree with the measurements to within 8%, suggesting that the kinetic rate coefficients and calculated NO 2 photolysis rate used in the SS approximation are reasonably accurate for conditions in the lower stratosphere. However, NO2 values observed in the Concorde exhaust plume were significantly less than SS values. Calculated NO2 photolysis rates showed good agreement with values inferred from solar flux measurements, indicating a strong self-consistency in our understanding of UV radiation transmission in the lower stratosphere. Model comparisons using a full diurnal, photochemical steady state model also show good agreement with the NO and NO2 measurements, suggesting that the reactions affecting the partitioning of the NOy reservoir are well understood in the lower stratosphere. Introduction Stratospheric ozone (03) is destroyed in catalytic cycles involving oxides of hydrogen (HO x = OH + HO2), nitrogen (NO x = NO + NO2), chlorine (C10 x = C1 + C10 + 2C1OOC1), and bromine (BrO x = Br + BrO). Our understanding of 03 destruction cycles changed significantly with the recognition of the role of heterogeneous reactions in the stratosphere
Journal of Geophysical Research, 2002
In this paper we demonstrate the utility of the Polar Ozone and Aerosol Measurement (POAM) III da... more In this paper we demonstrate the utility of the Polar Ozone and Aerosol Measurement (POAM) III data for providing semiglobal three-dimensional ozone fields during the Stratospheric Aerosol and Gas Experiment (SAGE) III Ozone Loss and Validation Experiment (SOLVE) winter. As a solar occultation instrument, POAM III measurements were limited to latitudes of 63°N to 68°N during the SOLVE campaign but covered a wide range of potential vorticity. Using established mapping techniques, we have used the relation between potential vorticity and ozone measured by POAM III to calculate threedimensional ozone mixing ratio fields throughout the Northern Hemisphere on a daily basis during the 1999/2000 winter. To validate the results, we have extensively compared profiles obtained from ozonesondes and the Halogen Occultation Experiment to the proxy O 3 interpolated horizontally and vertically to the correlative measurement locations. On average, the proxy O 3 agrees with the correlative observations to better than $5%, at potential temperatures below about 900 K and latitudes above about 30°N, demonstrating the reliability of the reconstructed O 3 fields in these regions. We discuss the application of the POAM proxy ozone profiles for calculating photolysis rates along the ER-2 and DC-8 flight tracks during the SOLVE campaign, and we present a qualitative picture of the evolution of polar stratospheric ozone throughout the winter.
Journal of Geophysical Research: Atmospheres, 1999
Stratospheric measurements of NO, NO2, 03, C10, and HO2 were made during spring, early summer, an... more Stratospheric measurements of NO, NO2, 03, C10, and HO2 were made during spring, early summer, and late summer in the Arctic region during 1997 as part of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) field campaign. In the sunlit atmosphere, NO2 and NO are in steady state through NO2 photolysis and reactions involving 03, C10, BrO, and HO2. By combining observations of 03, C10, and HO2, observed and modeled values of the NO2 photolysis rate coefficient (JNO2), and model estimates of BrO, several comparisons are made between steady state and measured values of both NO2 and JNO2. An apparent seasonal dependence in discrepancies between calculated and measured values was found; however, a source for this dependence could not be identified. Overall, the mean linear fits in the various comparisons show agreement within 19%, well within the combined uncertainties (+50 to 70%). These results suggest that photochemistry controlling the NO2/NO abundance ratio is well represented throughout much of the sunlit lower stratosphere. A reduction in the uncertainty of laboratory determinations of the rate coefficient of NO + 03-• NO2 + 02 would aid future analyses of these or similar atmospheric observations. 1. Introduction With mixing ratios in the parts per trillion (pptv) to parts per billion (ppbv) range in the lower stratosphere, hydrogen, nitrogen, and halogen radicals (HO x = OH + HO2; NOx = NO + NO2; C1Ox = C1 + C10; BrO x = Br + BrO) play important roles in the destruction of stratospheric ozone
Geophysical Research Letters, 2000
In situ measurements of radical and long-lived species were made in the lower Arctic stratosphere... more In situ measurements of radical and long-lived species were made in the lower Arctic stratosphere (18 to 20 km) between spring and early autumn in 1997. The measurements include 0 3, C10, OH, HO2, NO, NO2, N20, CO, and overhead 03. A photochemical box model constrained by these and other observations is used to compute the diurnally averaged destruction and production rates of 03 in this region. The rates show a strong dependence on solar exposure and ambient 03. Total destruction rates, which reach 19%/month in summer, reveal the predominant role of NOx and HOx catalytic cycles throughout the period. Production of 03 is significant only in midsummer air parcels. A comparison of observed 03 changes with destruction rates and transport effects indicates the predominant role of destruction in spring and an increased role of transport by early autumn.
Geophysical Research Letters, 1990
A:bstrac•. Simultaneous, in situ measurements of OH, :HOa, HaO, and O, from 37-23 km are reported... more A:bstrac•. Simultaneous, in situ measurements of OH, :HOa, HaO, and O, from 37-23 km are reported, The par-6fioning between OH and HOa and the total HO• concentm•:io.n are compared w•th expected steady-state values. The ratio of HOa to OH varies from less than 2 at 3õ km to .more than 3 at 25 km; in the lower stratosphere this ratio [s nearly a factor of two less than predicted. The data are used to calculate HO• production and loss rates. The measured HOx mixing ratio is consistent with production dominated by the reaction of O(•D) with HaO, and loss controlled by NOy below 28 km and HOx above 30 km. T/he steady-state concentration of H20• is inferred from t,he •me•ured HOa concentration and calculated photolym rate. The maximum H=Oa mixing ratio (at 33 km) is p•icted to be less than 0.2 ppb.
Twilight observations suggest unknown sources of HOx
Geophysical Research Letters, 1999
Atmospheric Chemistry and Physics, 2013
Measured upwelling radiances from Nimbus-7 SBUV (Solar Backscatter Ultraviolet) and seven NOAA SB... more Measured upwelling radiances from Nimbus-7 SBUV (Solar Backscatter Ultraviolet) and seven NOAA SBUV/2 instruments have been used to calculate the 340 nm Lambertian equivalent reflectivity (LER) of the Earth from 1979 to 2011 after applying a common calibration. The 340 nm LER is highly correlated with cloud and aerosol cover because of the low surface reflectivity of the land and oceans (typically 2 to 6 RU, reflectivity units, where 1 RU = 0.01 = 1.0 %) relative to the much higher reflectivity of clouds plus nonabsorbing aerosols (typically 10 to 90 RU). Because of the nearly constant seasonal and longterm 340 nm surface reflectivity in areas without snow and ice, the 340 nm LER can be used to estimate changes in cloud plus aerosol amount associated with seasonal and interannual variability and decadal climate change. The annual motion of the Intertropical Convergence Zone (ITCZ), episodic El Niño Southern Oscillation (ENSO), and latitudedependent seasonal cycles are apparent in the LER time series. LER trend estimates from 5 • zonal average and from 2 • × 5 • , latitude × longitude, time series show that there has been a global net decrease in 340 nm cloud plus aerosol reflectivity. The decrease in cos 2 (latitude) weighted average LER from 60 • S to 60 • N is 0.79 ± 0.03 RU over 33 yr, corresponding to a 3.6 ± 0.2 % decrease in LER. Applying a 3.6 % cloud reflectivity perturbation to the shortwave energy balance partitioning given by Trenberth et al. (2009) corresponds to an increase of 2.7 W m −2 of solar energy reaching the Earth's surface and an increase of 1.4 % or 2.3 W m −2 absorbed by the surface, which is partially offset by increased longwave cooling to space. Most of the decreases in LER occur over land, with the largest decreases occurring over the US (−0.97 RU decade −1), Brazil (−0.9 RU decade −1), and central Europe (−1.35 RU decade −1). There are reflectivity increases near the west coast of Peru and Chile (0.8 ± 0.1 RU decade −1), over parts of India, China, and Indochina, and almost no change over Australia. The largest Pacific Ocean change is −2 ± 0.1 RU decade −1 over the central equatorial region associated with ENSO. There has been little observed change in LER over central Greenland, but there has been a significant decrease over a portion of the west coast of Greenland. Similar significant decreases in LER are observed over a portion of the coast of Antarctica for longitudes −160 • to −60 • and 80 • to 150 • .
Applied Physics B Photophysics and Laser Chemistry, 1985
The reaction of NO2 with isobutane, induced by 488 nm laser radiation, to form 2-nitro-2-methylpr... more The reaction of NO2 with isobutane, induced by 488 nm laser radiation, to form 2-nitro-2-methylpropane has been investigated and the results computer-modeled according to two possible reaction mechanisms. The first scheme involves the direct abstraction of H from isobutane by vibronically excited NO2 (NO**), and the second, abstraction by an intermediate NO 3 radial produced by NO**+ NO2. The modeling results strongly support the NO** scheme as the dominant reaction mechanism.
Journal of Geophysical Research, 2003
The International Photolysis Frequency Measurement and Model Intercomparison (IPMMI) took place i... more The International Photolysis Frequency Measurement and Model Intercomparison (IPMMI) took place in Boulder, Colorado, from 15 to 19 June 1998, aiming to investigate the level of accuracy of photolysis frequency and spectral downwelling actinic flux measurements and to explore the ability of radiative transfer models to reproduce the measurements. During this period, 2 days were selected to compare model calculations with measurements, one cloud-free and one cloudy. A series of ancillary measurements were also performed and provided parameters required as input to the models. Both measurements and modeling were blind, in the sense that no exchanges of data or calculations were allowed among the participants, and the results were objectively analyzed and compared by two independent referees. The objective of this paper is, first, to present the results of comparisons made between measured and modeled downwelling actinic flux and irradiance spectra and, second, to investigate the reasons for which some of the models or measurements deviate from the others. For clear skies the relative agreement between the 16 models depends strongly on solar zenith angle (SZA) and wavelength as well as on the input parameters used, like the extraterrestrial (ET) solar flux and the absorption cross sections. The majority of the models (11) agreed to within about ±6% for solar zenith angles smaller than $60°. The agreement among the measured spectra depends on the optical characteristics of the instruments (e.g., slit function, stray light rejection, and sensitivity). After transforming the measurements to a common
Detailed Modeling and Analysis of the CPFM Dataset
A quantitative understanding of photolysis rate coefficients (or “j-values”) is essential to dete... more A quantitative understanding of photolysis rate coefficients (or “j-values”) is essential to determining the photochemical reaction rates that define ozone loss and other crucial processes in the atmosphere. )Values can be calculated with radiative transfer models, derived from actinic flux observations, or inferred from trace gas measurements. The principal objective of this study is to cross-validate j-values from the Composition and Photodissociative Flux Measurement (CPFM) instrument during the Photochemistry of Ozone Loss in the Arctic Region In Summer (POLARIS) and SAGE I11 Ozone Loss and Validation Experiment (SOLVE) field campaigns with model calculations and other measurements and to use this detailed analysis to improve our ability to determine j-values. Another objective is to analyze the spectral flux from the CPFM (not just the j-values) and, using a multi-wavelength/multi-species spectral fitting technique, determine atmospheric composition.
Final Report: Summary of Research for NASA ACMAP NAG511297 Detailed Modeling and Analysis of the CPFM Dataset Principal Investigator: Co-Investigators
Lunar Volvelles and Moondials in Baroque Germany
Journal for the History of Astronomy, 1989
Issues in Stratospheric Ozone Depletion
ABSTRACT
Modeling the Latitude-Dependent Increase in Non-Melanoma Skin Cancer Incidence as a Consequence of Stratospheric Ozone Depletion
Stratospheric Ozone Depletion/UV-B Radiation in the Biosphere, 1994
The principal medical concern of stratospheric ozone depletion is that ozone loss will lead to th... more The principal medical concern of stratospheric ozone depletion is that ozone loss will lead to the enhancement of ground-level UV-B radiation. The incidence of non-melanoma (basal and squamous cell) skin cancers is correlated with cumulative lifetime UV exposure. Global ozone climatology (40°S to 50°N latitude) was incorporated into a radiation field model to calculate the biologically accumulated dosage (BAD) of UV-B radiation integrated over days, months, and years. The slope of the annual BAD as a function of latitude was found to correspond to epidemiological data for non-melanoma skin cancers for 30°N to 50°N. Various ozone loss scenarios were investigated. It was found that that a small ozone loss in the tropics can provide as much additional biologically effective UV-B as a much larger ozone loss at higher latitudes. Also, for ozone depletions of >5%, the BAD of UV-B increases exponentially with decreasing ozone levels.
STARS: STellar Absorption and Refraction Sensor
Space 2004 Conference and Exhibit, 2004
ABSTRACT
<title>Optical remote sensing of ozone, clouds, and surface effects from the ER-2 platform during the NASA ASHOE/MAESA campaign</title>
Atmospheric Sensing and Modeling II, 1995
ABSTRACT
Advancing Our Understanding of the Atmosphere and Ionosphere Using Remote Sensing Techniques
any human activities affect or are affected by physical and chemical processes occurring in the E... more any human activities affect or are affected by physical and chemical processes occurring in the Earth's atmosphere, ionosphere, and near-Earth space environment. In this article, we review a wide range of remote sensing research activities being carried out within the APL Space Department that have provided a better understanding of these processes. The research, conducted from both the ground and
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Papers by Steven A. Lloyd